Synergistic binding sites in a hybrid ultramicroporous material for one-step ethylene purification from ternary C2 hydrocarbon mixtures

One-step separation of C2H4 from ternary C2H2/C2H4/C2H6 hydrocarbon mixtures is of great significance in the industry but is challenging due to the similar sizes and physical properties of C2H2, C2H4, and C2H6. Here, we report an anion-pillared hybrid ultramicroporous material, CuTiF6-TPPY, that has the ability of selective recognition of C2H4 over C2H2 and C2H6. The 4,6-connected fsc framework of CuTiF6-TPPY exhibits semi–cage-like one-dimensional channels sustained by porphyrin rings and TiF62− pillars, which demonstrates the noticeably enhanced adsorption of C2H2 and C2H6 over C2H4. Dynamic breakthrough experiments confirm the direct and facile high-purity C2H4 (>99.9%) production from a ternary gas mixture of C2H2/C2H6/C2H4 (1/9/90, v/v/v) under ambient conditions. Computational studies and in situ infrared reveal that the porphyrin moieties with large π-surfaces form multiple van der Waals interactions with C2H6; meanwhile, the polar TiF62− pillars form C–H•••F hydrogen bonding with C2H2. In contrast, the recognition sites for C2H4 in the framework are less marked.

In EXAFS fitting, the adjacent coordination elements in the periodic table are difficult to be precisely distinguished, as well as the Cu-N and Cu-F paths. For this reason, Cu is firstly only considered as one M-N shell, and the obtained results showed that the coordination number is close to 6. On this basis, Cu-F was introduced to limit the coordination number, the result of Cu2 and expected Cu-N4F2 was fitted. As shown in Table S1, the fitting parameters for the Cu-N4F2 coordination environment are reasonable.

Structure simulation
The structure model of CuTiF6-TPPY was generated, based on the coordination mode by XAS data, using the Materials Studio suite of programs. The unit cell structures (e.g., cell parameters and atomic positions) of CuTiF6-TPPY were calculated using the Forcite and Castep module. The Rietveld refinement, a software package for crystal determination from the XRD pattern, was performed to optimize the lattice parameters iteratively until the wRp value converges. The pseudo-Voigt profile function was used for whole profile fitting and Berrar-Baldinozzi function was used for asymmetry correction during the refinement processes. Line broadening from crystallite size and lattice strain were both considered.

Isotherm fitting
The pure-component isotherms of C2H2, C2H4 and C2H6 were fitted using single-site Langmuir-Freundlich model for full range of pressure (0~1 bar).

= 1 +
Here, p is the pressure of the bulk gas at equilibrium with the adsorbed phase (bar), q is the adsorbed amount per mass of adsorbent (mmol g −1 ), qsat is the saturation capacities (mmol g −1 ), b is the affinity coefficient (bar −1 ), and v represent the deviation from an ideal homogeneous surface.

IAST calculations
The selectivity of the preferential adsorption of component 1 over component 2 in a mixture containing 1 and 2 can be formally defined as:

Isosteric heat of adsorption
The isosteric heat of C2H2, C2H4, and C2H6 adsorption, Qst, defined as      As for C2H2 adsorption, the stretching band ν(-CH) of adsorbed C2H2 appeared at a lower frequency of ~2950 cm -1 besides the frequency at ~3260 cm -1 , which was induced by the formation of hydrogen bonding between C2H2 and TiF6 2anions. The asymmetrical C≡C stretching band appears at a lower frequency of ~1950 cm -1 rather than 2100~2140 cm -1 also confirmed the formation of hydrogen bonding between C2H2 and TiF6 2anions (56). For C2H6 adsorption, the stretching band ν(-CH) of adsorbed C2H6 appeared at the lower frequency of 2800~3075 cm -1 implies the interactions between C2H6 and TiF6 2anions. The stretching bands of β-(CH) and δ(-CH) of adsorbed C2H6 were detected without frequency shifts, indicating that C2H6 also interacted with the aromatic skeleton of CuTiF6-TPPY. As for C2H4, the δ (-CH) bending vibration bands appeared at ~1010 cm -1 and ~950 cm -1 belong to the absorption peak of RCH=CH2 (57,58), no other interaction can be observed. To check the dependence of these spectral changes on the loading amount of guest molecules inside MOFs, time-dependent in-situ IR spectra with C2 gas-loadings were conducted. As shown in Fig. S29, the characteristic absorption peaks of C2H2 (ν (-CH) and asymmetrical C≡C stretching) and C2H6 (ν(-CH)) appeared at 3 min. In contrast, the stretching bands of adsorbed C2H4 were detected at 10 min. These results indicated that C2H2 and C2H6 can be adsorbed in a stronger and faster manner than C2H4 by CuTiF6-TPPY.     This work